Land Grid Array (LGA) interposers provide an array of interconnections between a printed wiring board (PWB) and a chip module, such as a Multi-Chip Module (MCM). LGA interposers allow connections to be made in a way that are reversible and that do not require soldering as, for example, in ball grid arrays and column grid arrays. Ball grid arrays are unreliable on large areas because the lateral thermal coefficient of expansion driven stresses that develop exceed the ball grid array strength. Column grid arrays hold together despite the stresses but are soldered solutions and, thus, do not allow field replacibility of the chip modules. Field replacibility is important because it saves the customer significant cost in maintenance and upgrade of high-end computers for which LGAs are typically used.
Currently, there are at least two LGA types commercially available. Each suffers from serious problems. One type utilizes as the conducting medium a composite of siloxane elastomer and silver particles, which when mixed above the percolation threshold concentration, conducts electricity. The composite is injection molded into the shape of buttons across a full LGA in one injection mold operation, and is, thus, a good solution from the standpoint of manufacturing the LGA cost-effectively. Another advantage is that it can operate at low contact forces on the order of 30 to 80 grams per contact. Typically, current LGA's have several thousand contacts per interposer. A disadvantage is that the composite of elastomer and silver particles is itself a poor elastomer. In other words it undergoes significant plastic deformation under typical operating conditions and leads to eventual failure.
Another type of LGA interposer consists of an area array of button contacts each made of a random spaghetti coil spring. These springs are formed one at a time by injecting at high speed a length of gold-coated molybdenum wire into a mold. This interposer works reliably. However, they have a disadvantage of being expensive because of the button-by-button fabrication method. Also, they require being squeezed with a very high contact force between the MCM and the PWB. Forces on the order of 100 to 120 grams per button are typically required. Such high forces, once multiplied by the large number of buttons on an interposer, can cause deformation of the MCM and the PWB, and have been known to crack the chips mounted on the MCM. Further, these interposers do not scale reliably to larger MCMs with higher input/output (I/O) requirements. In addition, these interposers are also inconsistent with the introduction of organic packaging. Organic packages strive to replace expensive ceramic MCMs and offer better electric performance. Organic packages are much softer and more fragile and could thus only tolerate LGA forces much smaller than 100 to 200 grams per button.